Stimulus responsive nanoparticles

ABSTRACT

Disclosed are various embodiments of methods and systems related to stimulus responsive nanoparticles. In one embodiment includes a stimulus responsive nanoparticle system, the system includes a first electrode, a second electrode, and a plurality of elongated electro-responsive nanoparticles dispersed between the first and second electrodes, the plurality of electro-responsive nanorods configured to respond to an electric field established between the first and second electrodes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to copending U.S. provisionalapplication entitled “STIMULUS-RESPONSIVE FLUIDIC DISPERSIONS OFNANOPARTICLES” having Ser. No. 61/066,384, filed Feb. 19, 2008, which isincorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract/Grant No.NCC5-570, awarded by the National Aeronautics and Space Administration,Experimental Program to Stimulate Competitive Research (NASA WV EPSCoR).The Government has certain rights in this invention. This invention wasalso made with Government support under Contract No. NNX09CE79P, awardedby the National Aeronautics and Space Administration, and further wasmade with Government support under Contract No. NNG05GF80H, awardedthrough the Matt Shafran fellowship from WV Space Grant Consortiumfunded by NASA.

BACKGROUND

Liquid crystal monomers, also known as reactive mesogens, allow formolecular alignment to be controlled and for this alignment to becaptured through photo-polymerization to form densely cross-linked,ordered polymer networks. Reactive mesogen polymers may be used in arange of applications including, but not limited to, responsivemicrostructures for electro-optical switches, geometrical and opticalanisotropic colloids, thermally reactive polymer films, and opticalfilters.

SUMMARY

Embodiments of the present disclosure are related to stimulus responsivenanoparticles.

Briefly described, one embodiment, among others, includes a stimulusresponsive nanoparticle system. The system includes a first electrode, asecond electrode, and a plurality of elongated electro-responsivenanoparticles dispersed between the first and second electrodes, theplurality of electro-responsive nanorods configured to respond to anelectric field established between the first and second electrodes.

Another embodiment, among others, includes a method for orienting anelongated electro-responsive nanoparticle within a dispersion fluid. Themethod includes establishing an electric field between a first electrodeand a second electrode, the electric field extending through thedispersion fluid in proximity to the elongated electro-responsivenanoparticle and, responsive to the established electric field,reorienting the elongated electro-responsive nanoparticle within thedispersion fluid.

Another embodiment, among others, includes a method of fabricatingelongated electro-responsive nanoparticles. The method includes fillingan elongated electro-responsive nanoparticle template with reactivemesogen liquid crystal, curing the elongated electro-responsivenanoparticle template filled with reactive mesogen liquid crystal withUV light, etching away the elongated electro-responsive nanoparticletemplate, and separating the elongated electro-responsive nanoparticlesusing a sonicator.

Other systems, methods, features, and advantages of the presentdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the invention can be better understood with reference tothe following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 illustrates an exemplary method for fabricating stimulusresponsive nanoparticles according to one embodiment of the presentdisclosure;

FIG. 2 illustrates a parallel axial director and a biplanar director inthe stimulus responsive nanoparticles of FIG. 1, according to exemplaryembodiments of the present disclosure;

FIG. 3 is an illustration of a stimulus responsive nanoparticle system,including the stimulus responsive nanoparticles of FIG. 1, according toone embodiment of the present disclosure;

FIG. 4 illustrates an exemplary method for reorienting the stimulusresponsive nanoparticles of FIG. 1, according to exemplary embodimentsof the present disclosure;

FIG. 5 illustrates exemplary reorientations of elongatedelectro-responsive nanoparticles included in the stimulus responsivenanoparticle system of FIG. 3, according to one embodiment of thepresent disclosure; and

FIGS. 6A-6D illustrate exemplary reorientation and relaxation responsesof the elongated electro-responsive nanoparticles, such as thoseincluded in FIG. 5, according to one embodiment of the presentdisclosure.

DETAILED DESCRIPTION

Disclosed herein are various embodiments of methods and systems relatedto stimulus responsive nanoparticles. Reference will now be made indetail to the description of the embodiments as illustrated in thedrawings, wherein like reference numbers indicate like parts throughoutthe several views.

Photo-polymerization of liquid crystal monomers, also known as reactivemesogens, allows a molecular alignment to be captured by forming denselycross-linked, ordered polymer networks. Before polymerization,externally-applied fields can be used to manipulate the molecular orderto produce stimulus responsive nanoparticles such as, but not limitedto, elongated electro-responsive nanoparticles, which are responsive toelectric and/or magnetic field stimulation. Elongated electro-responsivenanoparticles may be utilized in a variety of applications such as, butnot limited to, electro-optical switches, optical filters, microfluidicdevices, micro-mixers, micro-viscometers, fluidic pumps and stirrers,flow sensors, and electrorheological and/or magnetorheological fluids.

FIG. 1 illustrates an exemplary method 100 for fabricating stimulusresponsive nanoparticles according to one embodiment of the presentdisclosure. Beginning with block 110, a template 150 is filled withreactive mesogen liquid crystal 160. In one embodiment, Anoporemembranes may be used as an elongated electro-responsive nanoparticletemplate to fabricate elongated electro-responsive nanoparticles. Insome embodiments, the Anopore membranes have a thickness in the range ofabout 20 microns to about 100 microns. For example, the thickness of theAnopore membrane is about 60 microns. In exemplary embodiments, theAnopore membranes have a pore diameter in the range of about 20nanometers to about 200 nanometers. Other embodiments may utilize othertemplates to provide other sizes and/or shapes.

The template 150 is filled with the reactive mesogen liquid crystal 160in the nematic state. In one embodiment, the reactive mesogen liquidcrystal 160 is RM257 (Merck). In other embodiments, reactive mesogenliquid crystals 160 can include, but are not limited to, RMM114 (Merck)and Paliocolor LC242 (BASF). In some embodiments, the reactive mesogenliquid crystal 160 may be mixed with a photoinitiator such as, but notlimited to, Daracur 1173 (Ciba Geigy Chemicals) to aid in curing. Thetemplate 150 (e.g., Anopore membrane) may be positioned to allow fillingby capillary action. Other methods of filling may also be utilized.After filling, excess reactive mesogen may be removed from the top andbottom of the template 150.

The nematic liquid crystal phase is characterized by molecules that haveno positional order but tend to point in the same direction along adirector. In some embodiments, the director of the liquid crystal alignsparallel to the pore surface in an untreated Anopore membrane. One suchconfiguration is the planar bipolar director. In other embodiments, thedirector of the liquid crystal aligns in a planar concentric directorconfiguration or a parallel axis configuration in an Anopore membrane.The configuration is a function of the elastic properties of the liquidcrystal monomer, the surface anchoring, and the presence of appliedelectric and/or magnetic fields. For example, as illustrated in FIG. 2,when confined in a cylindrical cavity the liquid crystal can adopt adirector field similar to a parallel axial director 210 or a planarbipolar director 220.

Referring back to FIG. 1, the reactive mesogen liquid crystal in thefilled template 150 is cured to form stimulus responsive nanoparticles170 in block 120. The reactive mesogen liquid crystal 160 is cured whilein the nematic range to capture the director field in the formednanoparticles 170. In one embodiment, the reactive mesogen is cured bypolymerizing in a UV curing chamber 180 while the temperature of theliquid crystal was kept between 71-74° C., which represents the lowerboundary of the nematic range of the liquid crystal. This temperaturecan maintain the ordering of the molecular orientation of the liquidcrystal parallel to the surface of the template. In other embodiments,temperatures are maintained between the melting/nematic phase transitiontemperature and the nematic/isotropic phase transformation temperature.The temperature during UV curing determines the order parameter and canbe used to tailor the anisotropy of the stimulus responsivenanoparticles. In one embodiment, an electric and/or magnetic field isapplied during curing to modify the director configuration.

After curing, the template 150 is removed from the stimulus responsivenanoparticles 170 in block 130. For example, the template 150 may beremoved by etching in a bath 190. In one embodiment, an Anopore membraneused as the template 150 may be etched away from the stimulus responsivenanoparticles 170 by immersing the cured template in a 0.5M NaOHsolution.

In block 140, the stimulus responsive nanoparticles 170 are separated.In one embodiment, the nanoparticles are separated using a sonicator(Branson 1510 R, 42 kHz) for a period of time (e.g., 15 minutes). Theseparated nanoparticles may then be dispersed in a nonconductive fluidor dispersion agent such as, but not limited to, silicone oil.

In the case of elongated electro-responsive nanoparticles, the shape ofnanorods can include, but are not limited to, cylindrical and prolatespheroid. In some embodiments, the cross-section of the elongatedelectro-responsive nanoparticles is non-circular (e.g., an oval). Inother embodiments, the cross-section of the elongated electro-responsivenanoparticles is an irregular geometric shape such as, but not limitedto, a trapezoid, pentagon, and hexagon.

In some embodiments, the length of the elongated electro-responsivenanoparticles can range from about 1 micron to about 100 microns. Inother embodiments, the length of the elongated electro-responsivenanoparticles can range from about 2 microns to about 60 microns. Inanother embodiment, the length of the elongated electro-responsivenanoparticles can range from about 10 microns to about 50 microns. Thediameter of the elongated electro-responsive nanoparticles can rangefrom about 10 nanometers to about 5 microns. In other embodiments, thediameter of the elongated electro-responsive nanoparticles can rangefrom about 20 nanometers to about 1 micron. In another embodiment, thediameter of the elongated electro-responsive nanoparticles can rangefrom about 20 nanometers to about 200 nanometers.

FIG. 3 is an illustration of a stimulus responsive nanoparticle system300 according to one embodiment of the present disclosure. The stimulusresponsive nanoparticle system 300 includes first and second electrodes,310 and 320 respectively. In the embodiment of FIG. 3, the first andsecond electrodes 310 and 320 include a conducting film, 312 and 322respectively, disposed on a surface of an insulating substrate, 314 and324 respectively. In other embodiments, the first and second electrodes310 and 320 include a conductive substrate. In alternative embodiments,the first and second electrodes 310 and 320 can include one or moreconducting regions within an insulating substrate.

Some embodiments of the electrodes 310 and 320 may include multiplelayers and/or regions. For example, one or more substrate layer may beused with one or more conducting layers. Alternatively, multipleconducting regions may be disposed on the same substrate to formmultiple electrodes in the same plane. In some embodiments, patternedelectrodes (e.g., one or more conductive trace, grid, or patterndisposed in a pattern on one or more substrate) may be included toestablish an electric or a magnetic field. The conductivity of thelayers may be classified as conducting, semi-conducting (includingn-type or p-type), or superconducting. In some embodiments, multiplelayers may be used to produce device layers and/or regions such as thinfilm transistors to actuate or establish an electric field.

The conducting film 312, conducting substrate, conducting layer and/orconducting region may be formed using conducting materials such as, butnot limited to, metals, conductive ceramic materials, and conductivepolymer materials. Metals can include, but are not limited to, silver,copper, iron, copper, and combinations thereof. Conductive ceramicmaterials can include, but are not limited to, indium oxides, tinoxides, zinc oxides, aluminum oxides, yttrium oxides, strontium oxides,and combinations thereof. Conductive polymers can include, but are notlimited to, polythiophenes, polyanilines, polypyroles and combinationsthereof. In other embodiments, conductive layers and/or regions caninclude semi-conducting organic materials such as, but not limited to,penatcene and carbon. In some embodiments, conductive layers and/orregions consist of conducting particles dispersed in a binder.

The first and second electrodes 310 and 320 may be transparent oropaque. In one embodiment, the electrodes 310 and 320 include an indiumtin oxide (ITO) film disposed on a glass substrate. Other layers mayalso be included in one or more electrodes to produce effects such as,but not limited to, optical effects, improved reliability, inducedorientation, and modified flow properties. In the embodiment of FIG. 3,the electrodes 310 and 320 are substantially planar. In otherembodiments, the electrodes may conform to other shapes orconfigurations such as, but not limited to, semi-circular or angled.

In the embodiment of FIG. 3, elongated electro-responsive nanoparticles330 are dispersed between the first and second electrodes 310 and 320.Elongated electro-responsive nanoparticles 330 include, but are notlimited to, cylindrical and prolate spheroid nanorods. In oneembodiment, the elongated electro-responsive nanoparticles 330 aredispersed in a nonconductive fluid or dispersion agent 340. Dispersionagents 340 can include, but are not limited to, silicone oil, de-ionizedwater, glycerol, corn syrup, and any other suitable nonconductive fluid.In the embodiment of FIG. 3, the elongated electro-responsivenanoparticles 330 and the dispersion agent 340 is sealed between thefirst and second electrodes 310 and 320.

In the embodiment of FIG. 3, the first and second electrodes 310 and 320are substantially parallel and separated by a distance equal to orgreater than a predefined length of the elongated electro-responsivenanoparticles 330 by spacers 350. For example, spacers with a diameterof about 60 microns may be used with elongated electro-responsivenanoparticles 330 with a length up to about 60 microns. In otherembodiments, the first and second electrodes 310 and 320 are separatedby a distance less than the longest length of the elongatedelectro-responsive nanoparticles 330. In one embodiment, the spacers 350are polymethacrylate spacers. Other materials suitable for use with thedispersion agent 340 and the electrode materials may also be used.

A voltage is provided to the electrodes 310 and 320 by a voltage supply360 to establish an electric field between the first and secondelectrodes 310 and 320. The voltage supply 160 can include, but is notlimited to, a DC voltage supply, an AC voltage supply, or some otherswitched or controlled voltage supply. In other embodiments, the voltagesupply 360 may be used to establish a magnetic field.

The orientation of elongated electro-responsive nanoparticles dispersedbetween electrodes may be altered by controlling the electric fieldestablished between the electrodes. For example, when a DC voltage isapplied to the first and second electrodes 310 and 320 of the embodimentof FIG. 3, an electric field is established between the first and secondelectrodes, and extends through the dispersion fluid in proximity to theelongated electro-responsive nanoparticles 330. The direction of theelectric field is determined by the polarity of the voltage supplied tothe first and second electrodes 310 and 320.

FIG. 4 illustrates an exemplary method 400 for reorienting the stimulusresponsive nanoparticles of FIG. 1, according to exemplary embodimentsof the present disclosure. In block 410, an electric field isestablished in proximity to the stimulus responsive nanoparticles. Withreference to the exemplary embodiment of FIG. 3, the electric field isestablished between the first and second electrodes 310 and 320. Theelectric field extends through the dispersion fluid 340 in proximity tothe elongated electro-responsive nanoparticles 330.

In response to the established electric field, the stimulus responsivenanoparticles are reoriented in block 420. FIG. 5 illustrates exemplaryreorientations of the stimulus responsive nanoparticles included in thestimulus responsive nanoparticle system of FIG. 3, according to oneembodiment of the present disclosure. In the exemplary embodiment ofFIG. 5, in response to establishing the electric field, elongatedelectro-responsive nanoparticles 520 with a biplaner director 220 (FIG.2) rotate 530 to align in parallel with the electric field asillustrated. In alternative embodiments, elongated electro-responsivenanoparticles 510 with a parallel axial director 210 (FIG. 2) translate540 toward the first electrode 310, while remaining perpendicular to theelectric field. Other reorientations may be possible.

A rotational response of elongated electro-responsive nanoparticles 520may modeled to calculate theoretical switching times. Electric torqueproduced by the electric field may be modeled as:T=|Δ∈|∈ _(o) E ² V cos θ sin θ  (1)where |Δ∈| is the dielectric anisotropy of a perfectly aligned nematic,∈_(o) is the permittivity of free space, E is the field strength, θ isthe angle of the elongated electro-responsive nanoparticle.

Torque as defined by mechanics may be modeled as:

$\begin{matrix}{{\omega\frac{\mathbb{d}\omega}{\mathbb{d}\theta}} = \frac{T}{I}} & (2)\end{matrix}$where the moment of inertia for a slender rod taken about the end of therod may be modeled as:

$\begin{matrix}{I = {\frac{1}{3}{ML}^{2}}} & (3)\end{matrix}$

Combining equations (1) and (2) and then integrating twice results in anequation relating time and angle.

$\begin{matrix}{{\Delta\; t} = {\sqrt{\frac{\rho\; L^{2}}{3{{\Delta\; ɛ}}ɛ_{0}E^{2}}}\left\lbrack {{\ln\left( {\tan\left( \frac{\theta_{2}}{2} \right)} \right)} - {\ln\left( {\tan\left( \frac{\theta_{1}}{2} \right)} \right)}} \right\rbrack}} & (4)\end{matrix}$where ρ is the density, L is the length, E is the field strength, θ₁ isthe initial angle of the elongated electro-responsive nanoparticle 520,and θ₂ is the final angle (e.g., 90°).

When the electric field is removed in block 430 (e.g., de-energizingvoltage supply 160), the stimulus responsive nanoparticles relax back totheir original orientation. Relaxation when the electric field is turnedoff and is driven by gravity and is relatively constant for thedifferent field strengths. In some embodiments, other particles may beincluded in the dispersion agent 340 to interact with the stimulusresponsive nanoparticles, such as the elongated electro-responsivenanoparticles 330, to amplify changes in viscosity and surface tensionand affect response times.

FIGS. 6A-6D illustrate exemplary reorientation and relaxation responsesof elongated electro-responsive nanoparticles, such as those included inFIG. 5, according to one embodiment of the present disclosure. Elongatedelectro-responsive nanoparticles 520 with a biplaner director 220 (FIG.2) were dispersed between electrodes 310 and 320 as illustrated in FIG.5. The elongated electro-responsive nanoparticles had a diameter ofabout 200 nanometers and a length of up to about 60 microns. Theelectrodes 310 and 320 comprise indium tin oxide (ITO) coated glassseparated by 60 micron spacers.

FIG. 6A is a graphical plot 610 depicting the typical angular varianceof an elongated electro-responsive nanoparticle during the cycle ofreorientation and relaxation with an application of 10 V. The switchingtime was driven by the electric field established between the electrodes310 and 320. The elongated electro-responsive nanoparticle was observedusing an optical microscope (Leitz LaborLux 12 Me, Mag. 20×). The angleof the elongated electro-responsive nanoparticle with respect to theglass plate plane was then calculated from the apparent length measured.The apparent length of the elongated electro-responsive nanoparticleduring reorientation and relaxation was measured using Matlab imageanalysis.

The reorientation 612 of the elongated electro-responsive nanoparticlewas initiated when the elongated electro-responsive nanoparticle wasparallel to the electrode plane (θ=0°) and finished when the elongatedelectro-responsive nanoparticle was perpendicular to the electrode plane(θ=90°). As shown in FIG. 6A, the re-orientation time 612 for theobserved elongated electro-responsive nanoparticle was approximately0.385 seconds as measured using a frame grabber (Guppy, Allied VisionsTechnology), which captured optical images every 0.055 seconds.

After the elongated electro-responsive nanoparticle was perpendicular tothe electrode plane (θ=90°), the voltage was maintained for a period 614to allow the position of the elongated electro-responsive nanoparticleto stabilize. The elongated electro-responsive nanoparticle was thenallowed to relax 616 by removing the electric field until the elongatedelectro-responsive nanoparticle returned to its initial position.

The observed reorientation time is compared to a predicted reorientationtime in the graphical plot 620 of FIG. 6B. Equation (4) was used tomodel the observed elongated electro-responsive nanoparticle, where ρ isthe density (1000 kg/m³), L is the length (30 μm), |Δ∈| is thedielectric anisotropy of a perfectly aligned nematic (2), ∈_(o) is thepermittivity of free space, E is the field strength, θ₂ is the finalangle (90°), and θ₁ is the initial angle (0°). This model assumesperfect alignment of the liquid crystal, no viscosity and uniformelectric field strength. In addition, a dielectric anisotropy value (Δ∈)of −2 was assumed, which corresponds to a perfect alignment of thedirector field. Calculating the effective dielectric anisotropy valuefrom the actual switching time data, leads to a |Δ∈| value of 2.43×10⁻⁴for a field strength of 10 V.

Observed reorientation times for different applied voltages (60 V, 40 V,10 V, and 5 V) are depicted in the graphical plot 630 of FIG. 6C. As canbe seen from FIG. 6C, the reorientation of the elongatedelectro-responsive nanoparticle is driven by the strength of theelectric field. The responsive nature of the elongatedelectro-responsive nanoparticle during reorientation is shown to reducewith a decrease of electric field strength. The fastest reorientationtime recorded, 0.11 seconds, is achieved with a field strength of 60 V(1 V/μm). The elongated electro-responsive nanoparticle remains activeat a relatively low voltage of 5 V (0.08 V/μm), however, this is thethreshold voltage. At this low voltage, the field strength is not strongenough to cause complete rotation of the elongated electro-responsivenanoparticle. As seen in FIG. 6C, the elongated electro-responsivenanoparticle only rotates approximately 75°.

FIG. 6D is a graphical plot 640 depicting the predicted reorientationtimes of the elongated electro-responsive nanoparticle with anapplication of different applied voltages (60 V, 40 V, 10 V, and 5 V).Again, the predicted reorientation times are faster than the measuredreorientation times. For example, at 60 V and 5 V, the predictedreorientation times are approximately 700 μs and 8500 μs, respectively.In comparison, the reorientation of the elongated electro-responsivenanoparticle at 5 V does not reach 90°.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations setforth for a clear understanding of the principles of the disclosure.Many variations and modifications may be made to the above-describedembodiment(s) without departing substantially from the spirit andprinciples of the disclosure. All such modifications and variations areintended to be included herein within the scope of this disclosure andprotected by the following claims.

1. A stimulus responsive nanoparticle system, comprising: a firstelectrode; a second electrode substantially parallel to the firstelectrode; and a plurality of elongated electro-responsive nanoparticleshaving a parallel axial director dispersed within a nonconductive fluidbetween the first and second electrodes, the plurality of elongatedelectro-responsive nanoparticles responsive to an electric fieldestablished between the first and second electrodes, where the pluralityof elongated electro-responsive nanoparticles are configured totranslate toward the first electrode while remaining substantiallyperpendicular to the established electric field.
 2. The system of claim1, wherein the plurality of elongated electro-responsive nanoparticlesare electro-responsive nanorods.
 3. The system of claim 2, wherein theelongated electro-responsive nanoparticles are reactive mesogennanorods.
 4. The system of claim 1, wherein the plurality of elongatedelectro-responsive nanoparticles are dispersed between the first andsecond electrodes in a nonconductive dispersion agent.
 5. The system ofclaim 1, wherein the first and second electrodes each comprise aconducting film on an insulating substrate.
 6. The system of claim 1,wherein the first electrode comprises a thin film transistor.
 7. Thesystem of claim 1, wherein the first and second electrodes aresubstantially planar.
 8. The system of claim 7, wherein the first andsecond electrodes are separated by a distance equal to a predefinedlength of the elongated electro-responsive nanoparticles.
 9. The systemof claim 7, wherein the first and second electrodes are separated by adistance less than a longest length of the elongated electro-responsivenanoparticles.